The present invention relates generally to materials useful in immunization of animals against infection by protozoan parasites of the genus Babesia (Order Piroplasmida). More specifically, the present invention provides a novel soluble antigen, specific for Babesia parasites, isolatable from the growth medium of Babesia-infected erythrocytes in cell culture and/or from merozoite stages of the parasite in the medium. Also provided are diagnostic reagents comprising said soluble antigen.
Babesia parasites are the causative agents of hemoclastic animal diseases on a world-wide basis. Tickborne species of this genus are known to infect: cattle (B. bovis, B. bigemina, B. divergens, B. major); dogs (B. canis, B. gibsoni, B. vogalia); horses (B. equi, B. caballi); and rodents (B. rodhaini, B. microti). B. bovis and B. microti are also known to be infective in humans.
To date, no immunologically active agent has been available for use as a vaccine component in conferring immunity against infection by Babesia parasites. This is so despite very substantial attempts by those skilled in the art to isolate and purify antigenic materials by available in vivo and in vitro techniques.
Many years of research directed toward isolation of antigenic materials from the blood of infected animals have yielded a number of serum isolates displaying some attributes of antigenicity. None, however, has been successfully employed to induce a protective, much less a long-term, anamnestic, response in recipient animals. Moreover, animals innoculated with antigenic material derived from the blood of infected animals generally respond with production of iso blood group antibodies, prohibiting any practical application for such materials as vaccines. For the most part, the most successful of in vivo preparative techniques have been useful only in providing rather crude diagnostic materials.
Failures of in vitro preparative technology have been due in large part to difficulties attending quantitative, in vitro propagation of Babesia parasites, and especially to the difficulties inherent in separating suspected immunologically significant materials (e.g., killed, whole or fragmented antigenic merozoite stages of the parasite) from host erythrocyte cells and "contaminating" host cell components such as hemoglobin. Parenthetically, problems in quantitative in vitro propagation of hemotropic protozoan parasites effectively preclude development of vaccines for a wide variety of parasitic diseases including malaria (Plasmodium) and anaplasmosis (Anaplasma). Among the more pertinent recent advances in the art of parasite propagation are disclosed in Trager, et al., Science, 193: pp. 673-675 (1976) and Speer, et al., Z. Parasitenk, 50: pp. 237-244 (1976). These references, respectively, describe Plasmodium propagation in erythrocytes maintained under greatly reduced oxygen tension and continuous Plasmodium propagation in selected eukaryotic host cells. The focus of the two above-mentioned publications is the development of extracellular antigenic parasites and parasite fragments for use as vaccine components.
The general unavailability of antigenic Babesia materials has correspondingly resulted in unavailability of specific diagnostic reagents for use in determining the presence of a Babesia disease state as well as the presence of Babesia antibodies in animal serum. Presently available reagents, as noted above, generally comprise crude preparations of antigenic materials obtained from the blood of infected animals.
There exists, therefore, a substantial need in the art for antigenic Babesia materials in a substantially pure form, which materials may be used as components of vaccines and diagnostic reagents.